It is planned to determine what factors affect the activity of adenylosuccinate synthetase, adenylosuccinate lyase and AMP- aminohydrolyase in vivo and if the three enzymes function as a cycle to control purine nucleotide levels and thereby exert control on metabolism in general. The enzymes will also be purified from liver and ascites tumor cells and a detailed kinetic study made of their mechanism of action, along with an investigation of their physical properties as they relate to enzymatic activity and function. An attempt will be made to determine the actual site of action of 6-mercaptopurine and how it effects its antitumor activity.